1. Field of the Invention
This invention relates to a method of monitoring the progress and pattern of a combustion or flame front being advanced through a combustible subterranean carbonaceous stratum. In particular, this invention relates to a method of monitoring both the vertical and lateral movement of an underground flame front. More particularly, this invention relates to a method of monitoring the pattern and spatial orientation of a flame front during in situ retorting of oil shale.
2. Description of the Prior Art
The term oil shale refers to sedimentary deposits containing organic materials which can be converted to oil shale. Oil shale contains an organic material called kerogen which is a solid carbonaceous material from which shale oil can be retorted. Upon heating oil shale to a sufficient temperature, kerogen is decomposed and a liquid product is formed.
Oil shale can be found in various places throughout the world, especially in the United States in Colorado, Utah and Wyoming. Some especially important deposits can be found in the Green River formation in Piceance Basin, Garfield and Rio Blanco counties, and northwestern Colorado.
Oil shale can be retorted to form a hydrocarbon liquid either by in situ or surface retorting. In surface retorting, oil shale is mined from the ground, brought to the surface, and placed in vessels where it is contacted with hot retorting gases. The hot retorting gases cause shale oil to be freed from the rock. Spent retorted oil shale which has been depleted in kerogen is removed from the reactor and discarded.
In situ combustion techniques are being applied to shale, tar sands, Athabasca sand and other strata in virgin state, to coal veins by fracturing, and to strata partially depleted by primary and even secondary and tertiary recovery methods.
In situ retorting oil shale generally comprises forming a retort or retorting area underground, preferably within the oil shale zone. The retorting zone is formed by mining an access tunnel to or near the retorting zone and then removing a portion of the oil shale deposit by conventional mining techniques. About 5 to about 40 percent, preferably about 15 to about 25 percent, of the oil shale in the retorting area is removed to provide void space in the retorting area. The oil shale in the retorting area is then rubblized by well-known mining techniques to provide a retort containing rubblized shale for retorting.
A common method for forming the underground retort is to undercut the deposit to be retorted and remove a portion of the deposit to provide void space. Explosives are then placed in the overlying or surrounding oil shale. These explosives are used to rubblize the shale and preferably form rubble with uniform particle size. Some of the techniques used for forming the undercut area and the rubblized area are room and pillar mining, sublevel caving, and the like.
After the underground retort is formed, the pile of rubblized shale is subjected to retorting. Hot retorting gases are passed through the rubblized shale to effectively form and remove liquid hydrocarbon from the oil shale. This is commonly done by passing a retorting gas such as air or air mixed with steam and/or hydrocarbons through the deposit. Most commonly, air is pumped into one end of the retort and a fire or flame front initiated. This flame front is then passed slowly through the rubblized deposit to effect the retorting. Not only is shale oil effectively produced, but also a mixture of off-gases from the retorting is also formed. These gases contain carbon monoxide, ammonia, carbon dioxide, hydrogen sulfide, carbonyl sulfide, and oxides of sulfur and nitrogen. Generally a mixture of off-gases, water and shale oil are recovered from the retort. This mixture undergoes preliminary separation (commonly by gravity) to separate the gases, the liquid oil, and the liquid water. The off-gases commonly also contain entrained dust and hydrocarbons, some of which are liquid or liquefiable under moderate pressure. The off-gases commonly have a very low heat content, generally less than about 100 to about 150 BTU per cubic foot.
One problem attending shale oil production in in situ retorts is that the flame front may "channel" through more combustible portions of the rubble faster than others. The resulting uneven passage of the flame can leave considerable portions of the rubblized volume bypassed and unproductive. Such channeling can result from non-uniform size and density distributions in the rubblized shale. If the shape of the flame front can be defined or packing variations detected within the retort, then channeling and its effects can be mitigated by controlling the air injection rate and oxygen content into various sectors of the retort, or by secondary rubblization if regions of poor density can be mapped.
A variety of prior art techniques have been established for determining the position and progress of underground combustion. These techniques range from indirect theoretical mathematical formulations on the one hand to rather simplistic direct measurements that can be done at the combustion site on the other. One example of the mathematical treatment can be found in a paper ("Locating a Burning Front by Pressure Transient Measurements," Paper No. SPE 1271) by Hossein Kazemi delivered at the October, 1965, Society of Petroleum Engineers Conference. Kazemi disclosed a method by which the distance from a measuring point to the combustion front could be calculated employing pressure transient measurements. In particular, the pressure fall-off observed at the bottom of the well hole in either injected liquid or in effluent gases could be related to the approach of a combustion front. Such pressure build-up and fall-off measurements were also described by H. K. Van Poolen in the Feb. 1, 1965 Oil and Gas Journal, Vol. 63, No. 5.
An equally elaborate technique was described by Dr. A. M. Feder in 1967 ("Infrared Sensing: New Way to Track Thermal Flood Fronts," World Oil (April 1967), p. 142) using an infrared system to locate subterranean thermal fronts by flying an infrared sensor over the investigated area. Thermal energy from a sub-surface heat source (combustion or steam-fronts) may be transferred to the terrain surface by conduction through the overburden formation, or by movement of heated water or gases to the surface via fractures. Infrared imaging would then be useful to identify the hot portions of the surface terrain. This method however is only a gross estimate of the position of an underground thermal front and does not yield reliable data on its depth, extent or movement.
Parker discloses in U.S. Pat. No. 3,031,762 the periodic measurement of the elevation of the ground at one or more points directly above the path of a combustion front until the ground at this point rises. Such a rise is interpreted to indicate the arrival of the combustion front directly under the elevated point. This method is dependent on the fact that combustion of a carbonaceous stratum causes an expansion of the stratum which is substantially immediately translated to a rise in the elevation of the ground surface directly over the expanded stratum. This method is uniquely applicable to combustion fronts which are primarily vertical and which move in a horizontal direction. Combustion fronts in the horizontal plane that propagate vertically would simply result in a roughly symmetrical elevated area with no information provided concerning the depth or speed of the front.
Parker also teaches in U.S. Pat. No. 3,072,184 a fuel pack in which separate masses of gas forming materials are spaced in the fuel pack at predetermined distances. Thus as the fuel pack burns it releases identifiable gases at spaced intervals which, when detected in the effluent gases, can be related to the progress of the combustion front in that particular fuel pack. This method is primarily useful in well bores and is not readily amenable to application in underground retorting.
U.S. Pat. No. 3,454,365 issued to Lumpkin et al discloses a method in which the gas from in situ combustion process is analyzed for its oxygen, carbon dioxide, hydrogen and hydrocarbon content. A small sample stream from the hot effluent during in situ combustion is treated, condensed and dried. It is subsequently analyzed to determine the relative concentrations of the various off-gases. This concentration level is then rationalized through a control computer which controls the air injection rate to maintain an optimum utilization of the oxygen in the air stream and to optimize the in situ cracking process. This process is directed primarily towards detecting the efficiency or effectiveness of the combustion process within the retort, and does not provide usable information concerning the speed, progress, extent or location of the flame front within the retort.
In U.S. Pat. No. 3,467,189, Dingley also employs a sample-and-analysis technique to detect the approach of a flame front. Physical properties such as the water to air ratio of the formation fluids which enter a production well are monitored, as well as the hydrogen ion concentration and the salinity of the water and the specific gravity of the liquid hydrocarbons. A signal indicating the close proximity of the combustion front to the production well is provided when limiting or static values are reached at the same time in any two of the physical properties of the formation fluids entering the production valve.
U.S. Pat. No. 3,483,730, issued to Gilchrist et al, employs thermocouples to monitor the change in temperature of the overburden near the ground surface at a plurality of points spaced around the point at which the combustion is initiated. These thermocouples respond to changes in temperature of the overburden during the heat movement of the underground combustion and thereby detect lateral movement of the flame front.
Related to the teachings of U.S. Pat. No. 3,483,730 is a method involving down-hole placement of temperature-sensing devices which indicate a sharp rise in temperature as the flame front arrives at the locus of the temperature-sensing device. One disadvantage in this method lies in the fact that the extremely high temperatures of the combustion front frequently destroy the temperature-sensing apparatus. Another disadvantage is in the cost of drilling holes to the formation level.
The technique of self-potential profiling, long used to locate mineral deposits, has recently been found to be useful as a tool for locating buried geothermal reservoirs. This technique involves the detection of small self-potential voltages which result from natural earth currents. Two metal stakes are placed in conductive ground and connected to a sensitive voltmeter which detects the generation of electromotive force in the surrounding rocks due to increases in temperature. The effective range of this method is somewhat limited and dependent upon a large area of thermal variation to generate a measurable voltage. In an underground retort however, very poor electrical coupling exists between the rubble and the retort walls. It is expected that any self-potential voltages generated within the retort will be poorly transmitted to the walls. Therefore, the self-potential voltages detectable by the surface sensors will be primarily those generated from the immediately adjacent retort walls-a much smaller thermal source than the entire flame front. This significantly reduces the efficacy of this method in underground retorting. Like the infrared imaging technique this method adequately detects the presence of thermal anomolies, but provides little information concerning the depth or movement of such thermal fronts.
Scientists at the Lawrence Livermore Laboratories have recently explored the use of high frequency electromagnetic probing to investigate underground anamolies. One application of the radiofrequency (RF) probing is to observe the progress of a burn front in the experimental underground coal gasification process. This technique involves lowering radio transmitters and receivers into bore holes drilled around the area of concern. Underground irregularities which have an effect on the passage of the RF waves can then be detected and located. Varying geological features, however, also affect the passage of the RF waves. In addition, underground water pockets, or any other interface causing a change in the dielectric constant, would also affect the passage of the RF waves. This method is therefore susceptible to interference caused by the presence of normal subterranean features.
It can be seen that the methods taught by the prior art are, in general, directed towards either (1) detecting lateral movement of a flame front, or (2) the vertical movement of a flame front, but not both. In addition, even those methods which are capable of detecting the directional movement and location of the front do not provide a means for ascertaining whether the front is tilted out of a desired orientation. Such tilts are undesirable as they can cause incomplete or inefficient combustion in the retort. In general, the prior art does not provide a means of detecting both the lateral and vertical location of a flame front, the speed with which the flame front is propagating through the carbonaceous stratum and the degree to which the front deviates from a desired horizontal or vertical plane. Once these parameters of the underground flame front are detected, various means can be employed to selectively speed up or hinder portions of this flame front to more efficiently effectuate the retorting process.
The general object of this invention is to provide a method of determining the progress and pattern of a combustion front in a carbonaceous stratum which avoids the aforesaid difficulties. A more specific object of this invention is to provide a method of determining both the vertical and lateral movement of an underground flame front. Another object of this invention is to provide a means of ascertaining the spatial orientation of the plane of an underground flame front.